1. Field of the Invention
This invention relates to lasers, and more particularly to an improved waveguide gas laser.
2. Prior Art
In a U.S. patent application entitled "WAVEGUIDE GAS LASER WITH HIGH FREQUENCY TRANSVERSE DISCHARGE EXCITATION", Ser. No. 869,542, filed for the inventor Katherine D. Laakmann on Jan. 16, 1978, and now U.S. Pat. No. 4,169,251 one applicant of the present invention disclosed a waveguide laser excited by means of a transverse discharge at RF frequencies in the range of about 30 Mhz to about 3 Ghz and in which a laser gas is disposed in an elongated chamber and discharge is established in the gas by means of an alternating electrical field applied to the chamber along a direction transverse to its length. The superior performance of that RF frequency transverse excited waveguide laser, as well as its reduced size and complexity in comparison to existing prior art such as that disclosed in U.S. Pat. Nos. 3,772,611 and 3,815,047, are specifically delineated in that patent application and need not be reviewed herein.
The present invention is also directed to a waveguide laser with high frequency excitation, but which provides certain substantial and novel improvements over applicant's prior invention. As a result, the present invention provides the many advantages of radio frequency transverse excitation waveguide lasers as disclosed in the aforementioned prior application, but in addition overcomes the difficulties and limitations of the aforementioned prior invention and thus provides a far more versatile and efficient laser device. One such improvement is the utilization of longitudinal excitation which renders the selection of waveguide bore size and electrode separation independent parameters and thus permits optimization of radio frequency selection independently of bore size.
An additional improvement of the current invention over the previously disclosed RF excited gas waveguide laser comprises a novel means for eliminating a serious and limiting disadvantage of the previously disclosed device, namely, the formation of non-uniformities or "hot spots" apparently resulting from the bistable discharge impedance characteristics. Such hot spots tend to decrease the gain, efficiency, and output power of the laser. To overcome this disadvantage of the prior art device, the present invention utilizes a combination of structural and circuit improvements including the use of series capacitive ballasting resulting in novel improvements in the waveguide structure. Additional improvement is achieved by means of a unique drive circuit that is used to sense the occurrence of low impedance hot spots and to adjust input power accordingly to minimize the otherwise non-uniform negative impedance characteristics.
Still an additional improvement is the use of a totally homogeneous structure which while furthering the remedy for the hot spot problem referred to above, results in a waveguide laser that is more compact, more stable with respect to gas chemistry and more reliable than would result merely from the teachings of applicant's prior invention related to a transverse RF excited waveguide laser.
Accordingly, the present invention has all of the advantages provided by applicant's prior device, namely, the high frequency transverse excitation waveguide laser, but in addition, the present invention overcomes the difficulties and limitations of the high frequency transverse excitation technique. The principal advantage of the longitudinal RF excitation over transverse RF excitation resides in the resulting independence in selection of active bore size of the laser chamber and electrode separation distance for the excitation means. Alternatively, the advantage resides in the opportunity to optimize the RF drive frequency independently of the bore size.
It is taught in the applicant's prior patent application that efficient lasers with RF excitation require an RF drive frequency to be sufficiently high so that electrons drift only a negligible distance in relation to the electrode gap separation during one half cycle of the alternating electric field. Otherwise, spacer-charge regions build up in the vicinity of the electrodes which results in higher electric field across the electrode gap and hence higher electron temperatures. It is also well known that in molecular gas laser physics the optimum electron temperature for maximizing laser head efficiency tends to be considerably lower than the electron temperature in self-sustained discharges. Furthermore, high electron temperatures lead to greater CO.sub.2 dissociation rates which lead to lower tube life. Therefore, in self-sustained discharges it is desirable to minimize the electron temperature and therefore, also minimize the electron field intensity required to sustain the discharge. Accordingly, for RF discharges there is a minimum RF drive frequency for maximizing laser head efficiency for a given electrode gap separation.
In a transverse RF excitation waveguide laser of the prior art, the laser bore size and electrode gas separation are inherently the same. Accordingly, the bore size dictates the minimum RF drive frequency and as the RF drive frequency is increased the coupling efficiency between the RF energy source and the discharge decreases and it becomes increasingly difficult to obtain efficient RF drive sources. Thus the overall laser efficiency for the transverse RF discharge of the prior art device suffers due to low laser head efficiency if the RF drive frequency is below the desirable minimum. On the other hand, if the RF drive frequency is too high the efficiency suffers because of the lowering in coupling efficiency between the RF source and the discharge.
The above-identified upper and lower RF frequency limitations become particularly problematical when the bore size is small and therefore requires a relatively high RF drive frequency in terms of laser head efficiency. Unfortunately, such high RF drive frequency results in lower coupling and power supply efficiency; a paradoxical situation. However, with the longitudinal RF excitation of the present invention and the resulting independence between electrode gas separation and bore size, it is now possible for the first time to choose the RF drive frequency for high coupling and power supply efficiency and then separately choose the electrode gap separation for optimum laser head efficiency.